Several different systems for calculating stardates have been used.
Prior to 2265, stardates were based on Earth's calendar. This system used the Earth calendar year as the base unit, with the day (numbered between 01-365/366) following the decimal. For example, Stardate 2233.04 was 4 January 2233, while Stardate 2259.246 was 3 September 2259.[Notes 1]
Sometime prior to the U.S.S. Enterprise NCC-1701's Five-Year Mission under Captain James T. Kirk in 2265, the stardate system changed, and was no longer linked to the Earth calendar. This system remained in use until the 24th century.
In 2322, another new stardate system was introduced. This married the previous two systems by having 1000 stardates be equivalent to one Earth year, but remained otherwise independent from the Earth calendar. Each stardate is approximately 8 hours, 45 minutes long, and is subdivided by a decimal; the number of digits following the decimal point can be used to calculate the precise time with great detail. The first day of each "stardate year" (when Stardate x999 becomes Stardate y000, where x and y represent the current "stardate years," e.g. 51999 and 52000) falls on 23 May of each Earth year; the First Contact Day holiday falls on Stardate x868 of each year (e.g. Stardate 54868.6 was 5 April 2377).[Notes 2]
Also in use during the 23rd and early 24th centuries was the Reference Stardate system. Unrelated to the other stardate systems, the Reference Stardates were also based on the Earth calendar and could be calculated retroactively, but were not widely used. Using the Earth year 2000 as a baseline, Reference Stardates marked centuries ahead of a forward slash, followed by two digits for the year, two for the month, a decimal, and the date (e.g. Reference Stardate 1/5104.16 for 16 April 2151, or -1/6609.08 for 8 September 1966).[Notes 3]
- Although the events of Star Trek (2009) took place within an alternate reality, the implication was that the timelines were identical prior to the changes wrought by Nero when he traveled back in time. The stardate system in use throughout the film clearly predated the changes to the timeline.
- The new stardate system introduced in Star Trek: The Next Generation followed much clearer rules than that of the original series. Each year was divided into 1000 stardates, and the Earth year 2364 was given in the season 1 finale, "The Neutral Zone;" this was the first time an Earth calendar year was mentioned on screen. It was not until "Homestead," in the seventh season of Star Trek: Voyager, when both an Earth date (First Contact Day, e.g., 5 April 2377) and a stardate (54868.6) were given in the same episode. This allows for a very precise calculation of which Earth dates align with a stardate.
- The Reference Stardate system introduced in FASA's Star Trek: The Roleplaying Game was an early effort to reconcile stardates with Earth calendar dates. This is now, in fact, of limited use as a reference, as the dates given were all based upon the Spaceflight Chronology book, which was almost entirely contradicted and superseded by the Star Trek Chronology. By cross-referencing known dates on both calendars, however, conversion is possible. Trekipedia has compiled a reference matrix, available on [Google Drive].
- "Q2." Star Trek: Voyager, Episode 265. Television. 11 April 2001.
- "The Cage." Star Trek, Episode 0. Television. 1965 (Unaired).
- Star Trek. Film. 8 May 2009.
- "Encounter at Farpoint." Star Trek: The Next Generation, Episodes 101-102. Television. 28 September 1987.
- "The Neutral Zone." Star Trek: The Next Generation, Episode 126. Television. 16 May 1988.
- "Homestead." Star Trek: Voyager, Episode 269. Television. 9 May 2001.
- Wheeler, Wm. John with McLimore, Guy W. Jr., Poehlein, Greg K., and Tepool, David F. "Cadet's Orientation Sourcebook." Star Trek: The Role Playing Game, Book 2004A. FASA Corporation, 1983.